Practical 2: Trilobites and Graptolites
The aims of this practical are as follows:
1.
2.
3.
4.
To teach you how to identify and describe trilobites and graptolites.
To enable you to understand these groups in the context of their mode of life.
To help you to begin to think about how classifications of fossils are achieved.
To introduce concepts relating to biostratigraphy.
At the end of it you should be able to
1.
2.
3.
4.
Draw and describe any trilobite or graptolite, identifying any unusual features.
Define to some extent the age of a rock containing one of these fossils.
Use taxonomic nomenclature with confidence and understanding.
Critically appraise biostratigraphic statements or data sets.
Assessment will be by exam, in the normal practical slots of week 5.
The practical is divided into four sections. You can do these in any order. Some
require specimens, while others are based on data included in this booklet.
Supplementary specimens and information are arranged on the long bench along the
side of the lab.
Section 1: Trilobites
You are provided with two trilobites. A demonstration of other specimens is laid out
on the side bench. For each of your two specimens you should prepare detailed
and careful, scaled and annotated drawings. You should also describe the fossil
in words, using the appropriate technical terms. You should be careful to
identify the view you are drawing, is it front, back, side, and so on. Also you
should be careful to include values in your description – for example, how many
segments, how long is the thorax. You should use pencil and draw large outlines,
rather than shading.
The purpose of these is to enable you to identify the fossil and to note any unusual
features which may be diagnostic of a particular age or life habit. Careful
observations are the key to palaeontology. Information is included in the following
part of the booklet to help you identify and interpret such features. For each of your
specimens you should attempt to identify the fossil to generic level and comment
on its mode of life (if you don’t know what generic means, leave this bit until you
have done section 3).
Trilobite 1:
Trilobite 2:
Glabella – a raised central area of the head
under which was the stomach.
A
.
Compound
eye
Head
or
Cephalon
Facial suture – a natural break
in the exoskeleton to facilitate
moulting
Segments –
below each
segment was
a limb/gill
pair.
Thorax
B.
Pygidium
Axial
lobe
Pleural lobe
Trilobites can be useful for dating rocks. They are entirely Palaeozoic, with greatest
abundance in the Cambrian through Carboniferous. Individual species can offer greater
precision.
C.
Cephalon
Thorax
Antennae
Pygidium
Hypostome,
mouth located
at rear. In
Calymene the
hypostome
was fixed to
the front of
the cephalon.
Central groove between legs.
Food was probably manipulated
by the legs into this groove and
then moved to the mouth. The
first leg segments are serrated
to provide gripping and tearing
functions.
Leg/gill pairs. One pair for each
segment in the thorax, probably
three under the cephalon and
several vestigial ones under the
pygidium.
D.
Figure 1. The main elements of trilobite morphology. A. Calymene, an Ordovician predatory trilobite
in dorsal view, showing the main, well calcified, elements of the carapace. Specimen 4 cm long. B.
Two views of the same animal enrolled, showing the tight fit that was made between the front of the
cephalon and the back of the pygidium. C. Calymene as it might have looked from below, showing
the lightly calcified or organic skeleton including the hypostome, legs and gills. D. The three main
adaptive strategies of trilobites away from a highly conserved body plan. Left, Trinucleus (3 cm long),
a blind trilobite with large frontal pitted region, which probably had a sensory function. Centre,
Agnostus (2 mm long), a tiny trilobite with much reduced thorax. Right, Selenopeltis (5 cm long), a
representative of the extremely spiny adaptation of trilobites.
Opipeuter (below) and Pricyclopyge
(right), two pelagic trilobites. This life
habit was characterised by large eyes
with good all round vision, and a
streamlined body form.
Calymene, an Ordovician predatory
trilobite, eating a worm. This trilobite
had a rigid hypostome fixed to the front
of the cephalon, and sharp spikes on the
limb segments for tearing prey.
Agnostus, a tiny Cambrian trilobite with a
highly derived morphology. This species
probably swam just above soft sediment and
fed using its body cavity to entrain water from
which the limbs removed food.
Bergamia, an Ordovician
blind trilobite characteristic
of deep water, where it
probably fed on suspended
food particles.
Olenus, a Cambrian trilobite that may have
been adapted to farming sulphate reducing
bacteria in low-oxygen conditions, a lifestyle
seen in modern organisms living near deep sea
vents and black smokers.
Ampyx, an Ordovician filter
feeder. Water was drawn
through the partly enrolled
body and food removed
with the limbs.
Proetus, an
Ordovician deposit
feeder, at the end of
its feeding trial.
Cybeloides, an
Ordovician
burrower, with an
elevated eye
designed to emerge
from the sediment
when the rest of the
animal was buried.
Figure 3. The life habits of trilobites. These examples should help you to identify the mode of
life of most common trilobites, based on diagnostic features of their anatomy.
Section 2: Graptolites
You are provided with two graptolites. A demonstration of other specimens is laid out on the
side bench. For each of your two specimens you should prepare detailed and careful,
scaled and annotated drawings.
The purpose of these is to enable you to identify the fossil and to note any unusual features
which may be diagnostic of a particular age or life habit. Information is included in the
following part of the booklet to help you identify and interpret such features. For each of
your specimens you should attempt to identify the fossil to generic level and comment
on its mode of life (if you don’t know what generic means, leave this bit until you have done
section 3).
At the end of this exercise you should be able to identify even broken fragments of graptolite
and comment meaningfully on them.
Graptolite 1:
Graptolite 2
A.
Theca
Nema
Thecal
bandages
Fusellar
increments
Sicula
B.
scandent
reclined
horizont
al
declined
pendent
C.
Figure 1. The main elements of graptolite hard-part morphology. A. Graptoloid (sicula is 2
mm long), B. Terms for describing the orientation of a graptolite stipe, C. Different thecal
types, left to right glyptograptid, dicranograptid, climacograptid, hooked monograptid,
involute.
Graptolites are brilliant fossils for dating rocks. The entire range of planktonic
graptolites is Ordovician to Devonian (Silurian in the UK).
In addition, monograptids are only found in the Silurian and Devonian, multiplebranched forms only in the Ordovician (beware of cladia-bearing forms).
Section 3: Taxonomy
All fossils have names, in fact a long series of names. Here is the set for us:
Phylum Vertebrata
Class Mammalia
Order Primates
Family Hominidae
Genus Homo
Species sapiens
This set of names reveals the fossil’s family tree and should represent a code for the
evolutionary relationships of the group. At one end of the scale is the phylum name.
Everything within a phylum shares a common ancestor and a common set of general
characteristics, such as symmetry elements or growth patterns. Each successive name
represents a smaller subset of organisms that also share a common ancestor with a more
precise set of related characteristics. The list also gives a sense of time. Vertebrates probably
evolved in the Cambrian, while the first mammals are Mesozoic in age and primates evolved
in the Cenozoic. You have encountered this idea already in Practical 1.
Check that you understand this by examining the family line in the diagram below. It shows
a series of branching lines representing evolutionary events that led to the animals named at
the top of that line. On this diagram draw a set of concentric lines to show what set of these
animals constitute each of the categories listed above. I have drawn in the last of these as an
example. Note the times, in millions of years, at which each group evolved.
At the other end of the scale from the phylum, the genus and species names identify one
unique member (the species) of a small group of closely related organisms (the genus).
These names are what you usually see on a fossil label, for example, Monograptus (genus)
priodon (species). Sometimes the name of the person who originally defined this species is
added to the name, for example, Monograptus priodon (Bronn). Genera should always be
written in italics or underlined and should begin with a capital letter. Species names should
always be written in italics or underlined and should begin with a lower case letter.
Make sure that the names of your fossils are correctly written. This seems a small
point, but it matters.
Section 4: Biostratigraphy
Biostratigraphy is the use of fossils to date rocks. Obviously, this is a relative date (younger
than, or older than), not an absolute date (432 million years). However, it can be a very
precise tool of great geological utility.
Here is how it works. Sets of fossils are collected from sedimentary sequences, and these
data are used to construct a set of ranges for individual species. These are then put together
to form a pattern of biodiversity for a period of time. Then any unknown section can be
compared to this reference and dated accordingly. Dating is done into units of time called
zones. These are of unknown length but their sequence is unique. Zones can be defined on
the first or last appearance of a useful fossil, or by a set of fossils, or even by the times when
a fossil was most abundant. Clearly each of these methods holds potential problems, and it is
a picky but useful thing to know about any particular zone.
Please take a few minutes to think about this problem. Below is a block diagram showing
how using the first appearance of a fossil can cause problems in dating. Can you
construct similar diagrams exemplifying the problems that may arise for each of the
other types of zone?
Cephalograptus cometa
Monograptus sedgwickii
Monograptus lobiferus
Monoclimacis limatulus
Monograptus triangulatus
Orthograptus insectiformis
Diplograptus magnus
Petalograptus retroversus
Rastrites longispinus
Monograptus fimbriatus
Petalograptus concinnus
Orthograptus cyperoides
Monograptus difformis
Pristiograptus fragilis
Coronograptus gregarius
Glyptograptus sinuatus
Rhaphidograptus toernquisti
Atavograptus atavus
Zones
Lagarograptus acinaces
Coronograptus cirrus
Glyptograptus incertus
Pristiograptus incommodus
Diplograptus longissimus
Climacograptus vesiculosus
Diplograptus modestus
Diplograptus diminutus
Climacograptus medius
Climacograptus normalis
Glyptograptus persculptus
Lower Silurian graptolite zones and species ranges for the British Isles
Ranges of species
sedgwickii
convolutus
argenteus
magnus
triangulatus
cyphus
acinaces
atavus
acuminatus
Here is a real problem in mapping complicated, faulted and lithologically monotonous units
in southern Scotland, which can be resolved using graptolite biostratigraphy. I have
simplified the problem, but it is a real one, fully explained in the BGS Memoir ‘Geology of
the Rhins of Galloway district’, edited by P. Stone.
On the facing page are the ranges of some Lower Silurian graptolites from the UK. Below is
the geological problem facing the Survey.
Rhins of Galloway
Silurian muds and greywackes with sparse horizons
containing graptolites. All contacts are faults.
Using the faunas preserved at each locality, work
out the graptolite zones to which each block
belongs.
A
Climacograptus normalis
Climacograptus medius
Diplograptus longissimus
Coronograptus cirrus
B
Coronograptus gregarius
Glyptograptus sinuatus
Petalograptus concinnus
Atavograptus atavus
H
A:
B:
C:
D:
E:
F:
G:
H:
G
A
C
Diplograptus magnus
Orthograptus insectiformis
Climacograptus normalis
Zones
H
Glyptograptus persculptus
Climacograptus medius
Diplograptus diminutus
Climacograptus normalis
F
B
C
D
D
Monograptus fimbriatus
Coronograptus gregarius
Atavograptus atavus
E
G
Lagarograptus acinaces
Pristiograptus incommodus
Climacograptus normalis
F
Orthograptus cyperoides
Rastrites longispinus
Coronograptus gregarius
Climacograptus normalis
E
Petalograptus retroversus
Orthograptus cyperoides
Monograptus lobiferus
Monograptus sedgwickii
Here you need to explore the consequences of what you have deduced.
1.
Give an example of a zone identification that you were confident about. What made this
a good set of fossils for this purpose?
2.
Give an example of a zone identification that you were less confident about. What made
this set of fossils less useful for dating rocks?
3. In general terms, can you list a set of criteria that make a particular fossil good or
bad for use in biostratigraphy?
4. In this example, what conclusions can you draw about the fault blocks? Are there likely
to be any repeated units, or is each sequence probably unique? Is there a general pattern
relating age to position in the fault set, for example, east-west or north-south. If so, what
is it?